<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>正方形</title>
    <style>
        .square {
            width: 10vw;
            height: 10vw;
            background: tomato;
        }
    </style>
</head>
<body>

<div class="square"></div>

</body>
</html>